From 6161d3162222f03eb8cf959b5ecc940c598a337e Mon Sep 17 00:00:00 2001
From: Dan Larkin-York <dan.larkin-york@mongodb.com>
Date: Thu, 1 Dec 2022 21:23:24 +0000
Subject: SERVER-71723 Store WriteBatch measurements using small_vector

(cherry picked from commit 7ca2aafe4560839b35d7712eb2887baab6c7711a)
---
 src/mongo/db/timeseries/bucket_catalog.cpp | 8 ++++----
 src/mongo/db/timeseries/bucket_catalog.h   | 8 ++++++--
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/src/mongo/db/timeseries/bucket_catalog.cpp b/src/mongo/db/timeseries/bucket_catalog.cpp
index 1d25e6036a6..396e58dddec 100644
--- a/src/mongo/db/timeseries/bucket_catalog.cpp
+++ b/src/mongo/db/timeseries/bucket_catalog.cpp
@@ -767,7 +767,7 @@ const BucketCatalog::BucketHandle& BucketCatalog::WriteBatch::bucket() const {
     return _bucket;
 }
 
-const std::vector<BSONObj>& BucketCatalog::WriteBatch::measurements() const {
+const BucketCatalog::BatchMeasurements& BucketCatalog::WriteBatch::measurements() const {
     return _measurements;
 }
 
@@ -797,9 +797,9 @@ bool BucketCatalog::WriteBatch::finished() const {
 
 BSONObj BucketCatalog::WriteBatch::toBSON() const {
     auto toFieldName = [](const auto& nameHashPair) { return nameHashPair.first; };
-    return BSON("docs" << _measurements << "bucketMin" << _min << "bucketMax" << _max
-                       << "numCommittedMeasurements" << int(_numPreviouslyCommittedMeasurements)
-                       << "newFieldNamesToBeInserted"
+    return BSON("docs" << std::vector<BSONObj>(_measurements.begin(), _measurements.end())
+                       << "bucketMin" << _min << "bucketMax" << _max << "numCommittedMeasurements"
+                       << int(_numPreviouslyCommittedMeasurements) << "newFieldNamesToBeInserted"
                        << std::set<std::string>(
                               boost::make_transform_iterator(_newFieldNamesToBeInserted.begin(),
                                                              toFieldName),
diff --git a/src/mongo/db/timeseries/bucket_catalog.h b/src/mongo/db/timeseries/bucket_catalog.h
index bc907eadccf..5880ca39dff 100644
--- a/src/mongo/db/timeseries/bucket_catalog.h
+++ b/src/mongo/db/timeseries/bucket_catalog.h
@@ -53,6 +53,10 @@ protected:
     static constexpr std::size_t kNumStaticNewFields = 10;
     using NewFieldNames = boost::container::small_vector<StringMapHashedKey, kNumStaticNewFields>;
 
+    // Number of measurements we can hold in a batch without needing to allocate memory.
+    static constexpr std::size_t kNumStaticBatchMeasurements = 10;
+    using BatchMeasurements = boost::container::small_vector<BSONObj, kNumStaticBatchMeasurements>;
+
     using StripeNumber = std::uint8_t;
 
     using EraCountMap = std::map<uint64_t, uint64_t>;
@@ -208,7 +212,7 @@ public:
          */
         const BucketHandle& bucket() const;
 
-        const std::vector<BSONObj>& measurements() const;
+        const BatchMeasurements& measurements() const;
         const BSONObj& min() const;
         const BSONObj& max() const;
         const StringMap<std::size_t>& newFieldNamesToBeInserted() const;
@@ -255,7 +259,7 @@ public:
         OperationId _opId;
         ExecutionStatsController _stats;
 
-        std::vector<BSONObj> _measurements;
+        BatchMeasurements _measurements;
         BSONObj _min;  // Batch-local min; full if first batch, updates otherwise.
         BSONObj _max;  // Batch-local max; full if first batch, updates otherwise.
         uint32_t _numPreviouslyCommittedMeasurements = 0;
-- 
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